In certain applications, it is desirable for powder coatings to have a surface that is smooth in appearance and has a high scratch resistance. Known is the use of wax agents to adjust the scratch resistance to the desired level. Examples for such waxes are hydrocarbon and fluorocarbon waxes. Such compositions often lead to coatings with a loss in technological properties, for example, reducing the gloss of the powder coatings. As a wax-containing coating is baked, the wax migrates to the coating/air interface where it forms a layer with reduced gloss. Shortcomings of this approach are that the wax may soften the coating surface and reduce its resistance to marring, staining and chemical attack. Furthermore, it is possible that the migrated wax will partially be washed from the coated surface.
It is known to improve the above-stated properties by incorporating hard particles into powder coating compositions. Such particles often cause undesired effect such as insufficient gloss and transparency of clear coatings.
Alternatively, nano-scaled particles having an average particles size in the range of below 100 nm can be used in clear or top coats. EP-A 1216278, EP-A 1195416 and DE-A 10239424 describe differently structured and functionalised nano-scaled particles and the use thereof in coatings to impart scratch resistance. Further, WO 03102089, DE-A 10241510, EP-A 1166283 and EP-A 1924655 disclose chemically modified nano-scaled particles, which can be congregated at the surface of the coatings, can be incorporated into the unsaturated monomers forming the binder, or can be particles based on an element-oxygen network with reactive and optionally, un-reactive functions on their surface. The nano-scaled particles need to be incorporated into the coatings without any undesired effect to the general range of properties such as optical quality, rheology, flow and adhesion, and they can have insufficient hardness and/or can cause also insufficient transparency of clear coatings.
Therefore, there is still a requirement to provide coatings with improved scratch resistance, without causing any undesired effect to the general range of properties of the coatings. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.